The present invention relates to a power supply apparatus having a fuel cell.
Currently, a secondary battery such as a lithium ion battery or a nickel metal hydride battery is used as a power supply apparatus in portable electronic/electrical equipment such as a personal computer. However, the secondary battery can consecutively supply power for only a maximum of about 4 hours to a personal computer. Recently, the fuel cell that can consecutively supply power for 20 to 40 hours to a personal computer is drawing attention.
It is circulation type fuel cell that is a representative type fuel cell which uses methanol for fuel. FIG. 11 is a block diagram showing the configuration of a circulation type fuel cell in the conventional art. In FIG. 11, reference numeral 111 denotes a return pump, reference numeral 112 denotes a dilution tank, reference numeral 113 denotes a methanol pump, reference numeral 114 denotes a methanol tank, reference numeral 1105 denotes a fuel cell control section, reference numeral 116 denotes a fuel cell, and reference numeral 1107 denotes a gas-liquid separator. The fuel cell 116 has a stack 122, a fuel pump 123, and an air pump 124.
Methanol (CH3OH) of a several to a 100% concentration is stored in the methanol tank 114.
The methanol pump 113 pumps methanol into the dilution tank 112 from the methanol tank 114 based on a command by the fuel cell control section 1105. The dilution tank 112 dilutes methanol of a several to a 100% concentration to a 5% wt methanol. The fuel pump 123 pumps diluted methanol into the stack 122 from the dilution tank 112 based on a command by the fuel cell control section 1105. The air pump 124 pumps air into the stack 122 based on a command by the fuel cell control section 1105.
In the stack 122, methanol is supplied to a fuel electrode (−), and air is supplied to an air electrode (+). At the fuel electrode (−), in the area referred to as a three-phase interface where methanol and water which are reactants, a catalyst (an electrode surface), and electrolyte comes in contact, methanol reacts with water and turns into carbon dioxide, hydrogen ions, and electrons (CH3OH+H2O→CO2+6H++6e−). Hydrogen ions pass across a polymer membrane, while electrons pass across an external load, and respectively reach the air electrode (+). At the air electrode (+), atmospheric oxygen comes in contact with hydrogen ions at the three-phase interface, deprives electrons from the catalyst (an electrode surface) and reacts, turning into water (3/202+6H++6e−→3H2O).
The stack 122 discharges the 3 to 5% wt methanol that is spent, carbon dioxide, and water from the fuel electrode (−) side. The stack 122 discharges water and air from the air electrode (+) side. The gas-liquid separator 1107 isolates carbon dioxide from the gas comprising methanol, carbon dioxide, and water which are discharged from the stack 122, and discharges it. The return pump 111 pumps the remaining isolated methanol and water into the dilution tank 112. Methanol and water are reused for producing a diluted methanol in the dilution tank 112.
A fuel cell system and a fuel cell control method of the conventional art which operates at the operating point with the highest energy conversion efficiency is disclosed in the Official Gazette of Japanese Unexamined Patent Publication No. 2000-12059. FIG. 12 is a block diagram showing the configuration of the fuel cell system of the conventional art. A reformer 1228 produces a hydrogen-rich gas (a reforming gas) containing hydrogen by the steam reforming reaction of methanol, from methanol and water that was poured in as fuel 1224. The fuel cell 1236 generates power using this hydrogen-rich gas as a fuel gas. The Control section 1220 derives a characteristic of output current-output voltage that corresponds to the quantity of gas taken in, and calculates from that characteristic the highest point of energy conversion efficiency in the fuel cell 1236. The fuel cell 1236 is operated at this point.
In the conventional circulation type fuel cells, there is a disadvantage that a considerable quantity of methanol is exhausted together with carbon dioxide, since it is difficult to divide only the carbon dioxide from spent fuel. Therefore, 10% of the quantity of supplied methanol at most is transformed into an effective power. Hence, the fuel utilization ratio is small.
The fuel cell system and the fuel cell control method in the conventional art have a disadvantage that the apparatus becomes expensive and upsizes, since it has a reformer. In the conventional art, the fuel cell 1236 is operated at the best point of energy conversion efficiency (=power generation efficiency×gas utilization ratio). Due to necessity of sufficiently supplying fuel to the fuel cell system, if the fuel cell system and the fuel cell control method of the conventional art were applied to a non-circulation type DMFC (Direct Methanol Fuel Cell), a large quantity of unused methanol would be discharged. Therefore, the fuel cell system and the fuel cell control method of the conventional art are not suitable for a non-circulation type DMFC.
A non-circulation type fuel cell as a type of the fuel cell that discharges spent fuel without circulating fuel is known. In the fuel cell, methanol supplied from the entry port of the fuel cell is gradually consumed, and is discharged from the exit port. However, there is a disadvantage that the output voltage of the fuel cell will drastically drop when supplied methanol is insufficient in contrast to the current to be outputted. In the conventional non-circulation type fuel cell with a view to making the fuel cell output the power with stability and deal with the sudden changes of load, substantial quantity of unused methanol is discharged from the fuel cell. However, methanol cannot be discharged as it is because of its toxicity. Since unused fuel is discharged to some extent, non-circulation type fuel cell was thought as not suitable for a fuel cell using toxic methanol for fuel.
Furthermore, the fuel cell system and the fuel cell control method of the conventional art has a characteristic of output current-output voltage of the initial period on a ROM in advance as a standard curve characteristic, and by reading this out, the highest point of energy conversion efficiency is calculated. Therefore, there is a disadvantage that the fuel cell cannot generate power that is targeted, when operating temperature deviates from an ideal value, or when the highest point deviates due to various factor such as the secular change of the fuel cell.
The present invention is made in view of these disadvantages mentioned above, and is intended to provide a stable power supply apparatus with a clean discharge and outstanding fuel utilization ratio, even when the fuel cell undergoes a secular change, or goes under various conditions such as temperature of a fuel cell or quantity of water of the electrolysis membrane of a fuel cell.